1. Field of the Invention
This invention relates in general to solar energy collectors for capturing solar energy falling over a surface area, in terms of heat, raising the temperature of a fluid, and particularly to a sun tracking solar energy collector system means for increasing the temperature of the fluid and the efficiency of solar energy capture per unit of area using focussing means, solar tracking means, solar cells and an internal type heat exchanger means.
2. Description of the Prior Art and of Optimizing Factors
There has been a number of devices for capturing and utilizing heat energy from the sunlight. One type of such devices is the passive non tracking solar energy collecting panels providing in effect heat exchanging means between a highly light absorbing coating and water which is used for heating during the winter and for faucet hot water the year around. Coatings with much higher absorbitivity than emissivity in the range of the solar spectrum have been developed. The principle of what is commonly known as the "hot house effect" whereby visible light is easily transmitted through a transparent sheet, such as glass or acrylic plastic, imparting heat energy onto the absorbing surface at relatively low temperatures, so that the surface then can only radiate in the infrared spectrum which is only partially transmitted through said sheet, is beneficially used to retain the captured heat. The main drawbacks of this type of solar collecting devices are:
1. Lacking sun tracking the surface where the light collecting device is installed is inefficiently used due to the cosine law, capturing only a small portion of its potential energy capture.
2. The temperature at which the water can be raised is relatively low (about 145.degree. F.). While low temperature is useful for heating and for hot water, it cannot be used efficiently for the generation of electricity or to provide for air-conditioning through an absorption type refrigeration unit.
3. Despite the "hot house effect" a considerable amount of the captured heat is being re-radiated into space, or lost through conduction to the outside air.
4. Defining a price quality factor as the yearly dollar benefit derived by the device, divided by the overall cost of the installation, it has been found that such factor is relatively low for this type of solar collectors.
A second type of solar collecting devices may be referred to as line focus solar energy collecting devices which use optical means such as Fresnel lenses or cylindrical paraboloidal reflectors to concentrate the sun light along a narrow focal line. This type may or may not include sun tracking means.
The main benefit of the line focus solar collectors is that they provide sufficient light concentration to heat working fluid such as water to an intermediate temperature such as between 200 and 300.degree. F. Such water temperatures are capable of efficiently providing heat to an absorption type air conditioning system besides providing heating during the winter and hot water.
The main drawback of the line focus solar collectors is the relatively high percentage or radiation losses, as a relatively high percentage of the heat exchanger receiving the radiation is exposed to radiate back into space. Difficulties also exist in keeping a vacuum between an inner heat exchanger and an outer shield when a slot along the outer shield must remain transparent for the solar radiation to enter. Attempting to cover the slot with a transparent substance such as glass while keeping the glass sealed surrounding a metallic inner heat exchanger, and at a wide range of temperatures, while maintaining a vacuum between the glass shield and the metallic heat exchanger.
A third type of solar collecting devices may be referred to as superimposed focusing collectors providing focusing of a multiplicity of sun tracking reflectors commonly referred to as heliostats, on to same area of a heat exchanger, where substantially high temperature can be sustained.
The main advantage of the superimproved focussing solar collectors in that they can gather substantial amounts of solar energy per day, measured in megawatt hrs. per day. A drawback is that only a small portion of the heliostats is contributing efficiently to the common heat exchanger at any time. Also because of the high temperatures involved at the heat exchanger radiation losses can be considerable. This type of system can be acceptable in areas with high intensity of sun-light and inexpensive land.
While the solar energy now falling over the roofs of houses and industrial buildings is almost entirely being wasted, as the price of oil will increase, the value of this energy will also increase. Solar energy collectors which may appear to provide a benefit in terms of hot water to a homeowner today may be determined to be wasteful a few years from now when the cost of thermal energy will have substantially been increased.
It should be noted that it is not only the number of BTU's collected per year per unit area of surface that counts but also the temperature at which these BTU's are counted. The higher the temperature of the working fluid the higher the value of the energy because the heat in the hotter working fluid can be converted into more valuable forms of energy such as electricity, at a higher efficiency. From Carnot's cycle considerations the maximum thermal efficiency possible in a heat engine is EQU nc=1-T.sub.L /T.sub.H ( 1)
Where T.sub.H and T.sub.L are the high and low temperatures between which the engine operates, expressed as absolute temperatures. If T.sub.L is taken to be the ambient temperature, which is usually the case, the efficiency n.sub.c deteriorates as T.sub.H is approaching T.sub.L. While n.sub.c is unattainable in practical engines, the expression (1-T.sub.L /T.sub.H) turns out to be a controlling factor in the expression of the efficiency of heat engines. It is for this reason that BTU's at relatively low temperature gases such as the gases from the flute of oil burners and the exhaust of the automobiles are allowed to excape because the temperature of such gases is too low compared to the high temperature of the system.
Another desirable feature of a solar energy collection system is the evenness of energy output during a normal sunny day. The smaller the variation of energy output during the day the less energy storage will be required by the system.
On these basis it will be useful that a qualification factor "q" of a solar energy collection system be defined in such a way as to include all relevant factors, such as shown in the following equation: EQU q=(1-T.sub.A/ T.sub.H).times.K /(A.times.C.times..sigma.) (2)
Where
T.sub.A is the average absolute ambient Temperature; PA1 T.sub.H is the high temperature of the working fluid; PA1 K is the ratio of the amount of energy retained by the collector divided by the amount of energy incident per unit area at normal incidence; PA1 A is the area of ground or roof utilized by the solar collecting device; PA1 C is total cost in dollars for implementing the system; and PA1 .sigma. is the standard deviation of BTU output per hour per unit area of collector during the 12 sunny hourly intervals, 6 a.m. to 6 p.m. daily. PA1 K is being optimized by the invention's providing means for sun tracking for both daily motion of the sun and the seasonal declination of latitude .+-.23.5.degree.. K is further being optimized by the invention's providing means for minimizing radiation losses. (1-T.sub.A /T.sub.H) is being improved by the invention increasing the temperature of the working fluid through focussing of the solar energy, preferably through lenses; PA1 A is being minimized by the invention by positioning a multiplicity of focussing lenses on a single platform thereby tracking the sun in unison and precluding shading of one lense by another as it usually occurs in other systems during the early morning and late afternoon hours. PA1 .sigma. is being minimized by the sun tracking and by avoiding shading through the single platform principle. PA1 optical focusing means for concentrating sunlight falling on rectangular surface strips of optical focusing elements onto a much smaller area in the vicinity of the focus of the optical focusing means: PA1 a tubular metallic heat exchanger for containing a working fluid such as water or a gas and running along the foci of the optical focusing means thereby transferring the heat received by the concentrated sunlight to the working fluid; PA1 an insulating tube preferably metallic placed around the heat exchanger and presenting a highly reflective internal surface towards the metalic heat exchanger for reflecting back radiation emitted by the heat exchanger; PA1 small circular openings on the insulating tube covered with a transparent material such as glass for allowing the focused sunlight to enter the insulating tube and to reach the heat exchanger, the apertures constituting only a small portion of the total surface of the insulating tube; PA1 a vacuum maintained between the heat exchanger and the insulating tube for lowering conductive heat losses between the heat exchanger and the outside air; and PA1 a solar tracking platform for supporting the solar focusing means, the heat exchanger, and the insulating tube so that the optical axis of the optical focusing means remains substantially alligned with the direction of the sun.